The need for steerable auxiliary axles on various types of vehicles (trucks, trailers, etc.) is well known. By the term "auxiliary" axle, as used herein, is meant an axle which is not the axle primarily used for steering the vehicle. Instead, an "auxiliary" axle is one usually added to the vehicle so as to be either forward or rearward of the primary axles and rearward of the vehicle's primary steering axle (e.g. in a straight truck such as a dump truck, garbage truck, etc.). The purpose of adding such an auxiliary axle to the vehicle is normally to increase the load carrying capacity of the vehicle and/or to satisfy highway weight limit laws. Such auxiliary axles may be of the non-liftable or liftable type as contemplated herein.
To render a wheeled axle bearing suspension system "steerable", the caster angle (from the vertical) of the axle (or as may be used at times synonymously in the art, the caster angle of the wheels) must be appropriately chosen to make the wheels of the suspension steerable. A recognized, but not necessarily required, caster angle (from the vertical) is from about 3.degree. to 5.degree.. Thus a 5.degree. caster angle, for example, may be employed for steerable forward travel, while for reverse travel the caster must then be changed, for example, to a -3.degree. caster angle from the vertical. In short, a caster angle for steerability in one travel mode does not normally render steerable the wheel in the opposite travel mode. A need thus arises in the art to provide a mechanism for adjusting (changing) the caster angle, thereby to allow the axle (e.g. auxiliary steerable axle) to be steered during either the reverse or forward mode of travel.
A few attempts have been made in the past to provide adjustable mechanisms whereby the vehicle (e.g. heavy duty truck or semitrailer) operator could change the caster of his auxiliary axle. Examples include those mechanisms disclosed in U.S. Pat. Nos. 4,770,430; 4,881,747; and 5,015,004. Such mechanisms have characteristically employed air cylinders whose extendable/retractable connecting rod for changing caster must be connected in such a way that they, unfortunately, become substantial load bearing members during normal and abnormal vehicle operation. Moreover, such mechanisms have often been quite complex, and in order to attempt to minimize the load which they must carry due to their design, particularly by the rod when extended, the stroke length of the rod has to be materially lengthened, thus resulting in the need for a large, heavy and expensive air cylinder.
For several years now, the heavy duty trucking art has known of a particularly advantageous type of suspension system known as a "parallelogram" suspension. An excellent and successful example of such a system is found in U.S. Pat. No. 5,403,031. Therein, a unique liftable axle suspension system is disclosed which employs not only a very advantageous form of a parallelogram arrangement, but provides a unique lifting mechanism for selectively raising and lowering the wheels of a vehicle from the road surface (i.e. the ground or other travel surface of the vehicle). This patent also teaches a manual caster adjustment feature for adjusting the caster angle of the axle (wheel) when the need arises.
The unique suspensions of this aforesaid U.S. Pat. No. 5,403,031 (commonly owned and having an overlapping inventorship herewith) constitute a principal background from which this invention arose. As to certain embodiments of this invention, they may be considered specific improvements upon this aforesaid patent in that there is herein provided a unique and effective caster adjusting mechanism which may be used instead of the caster adjustment disclosed in this prior patent, the same basic parallelogram structure being otherwise maintained so as to couple its unique efficacy with that of this invention. For this reason the entire disclosure of U.S. Pat. No. 5,403,031 is incorporated herein by reference.
Given this current state of the art, it is apparent that there exists a need for an axle suspension system, preferably of the advantageous parallelogram type, which has a more effective caster adjusting mechanism associated therewith, which is easily operated, even in a field location, automatically, and which does not create undue loads on the mechanism. It is a purpose of this invention to fulfill this and other needs which will become more apparent to the skilled artisan once given the following disclosure.